You can hook into the request
filter and set the post types to whatever you wish:
add_filter( 'request', 'cpt_in_rss' );
function cpt_in_rss( $request ) {
if ( isset( $request['feed'] ) && ! isset( $request['post_type'] ) ) {
$request['post_type'] = [ 'post', 'cpt1', 'cpt2' ];
}
return $request;
}
Here’s a trac ticket about what you are asking.